Yeasts are both important pathogens and members of natural, non-pathogenic communities. Effective control of pathogenic yeasts is dependent on our knowledge of the natural communities as a source of the pathogen and as a source of genetic variation within pathogenic microbial populations. An impediment to the understanding of pathogen/natural population interactions is the large gap between what is known about pathogenic yeasts and about natural populations of yeast. This grant will increase our knowledge of the evolutionary history of a natural yeast community which both serves a model for natural yeast communities and contains several species of yeast that are human pathogens. To accomplish this, I will use genetic variation (measured across the genome either as RAPD (randomly amplified polymorphic DNA) or AFLP (anonymous fragment length polymorphic) variation or both) to construct phylogenies of both related and unrelated cactophilic strains. The phylogenies will be correlated with patterns in physiological profile, geographic origins of the strains, host plant type, and killer/sensitive phenotype in order to trace the evolutionary history of the community. I will be able to determine if the community has evolved as a unit or represents several separate communities assembled within the cactophilic habitat at different times and places. In addition, I will assess effect of sources of error that are specific to the kinds of genetic data being collected on the ability of standard cladistic techniques to accurately uncover the true phylogenetic relationships among strains. I will do this through experiments designed to assess the level of error in the data and a simulation model of the effect of various levels and sources of error on the likelihood of finding the true cladogram. Students involved in the project will learn to identify yeasts; to do genetic and phylogenetic analyses; and to organize, analyze, and present their data. The interdisciplinary aspects of the project will integrate concepts presented in different courses and will widen the students' perspectives in biology and, consequently, encourage them to widen their perspectives when choosing a career in biology.